Digital optical density for measuring the difference in optical density between an absorber of known density and a sample absorber

ABSTRACT

An apparatus is provided for determining a difference in optical density between an absorber of known density and a sample absorber. An exponential decaying voltage is connected in series but of opposite polarity to a voltage source which is proportional to photodetector current flowing in a memory load. The current is generated by light of a specified wavelength passing through a standard optical absorber. The instant when the magnitude of the exponential voltage is equal to that generated by the photodetector, i.e., when the voltage across the combination is zero, the memory load is adjusted so that a specified number of clock pulses, No, (fixed by the clock frequency and the time constant of the exponential voltage) are counted by a ripple counter. A sample of unknown optical density at a specified wavelength is then measured, providing a voltage proportional to a selected calibrated memory standard. At the instant of time when this voltage, in combination with the exponential voltage is equal to zero, the difference in the optical density between the unknown and standard absorbers is equal to the absolute value of the difference between the reference count No and the count in the ripple counter. The density information is gated into a decade counter and read out digitally. Circuitry is provided for indicating positive or negative difference in density, and a reset automatically recycles the device at the end of a measurement. The density difference measurement is not affected by absolute light intensity or absolute photodetector sensitivity. This device is particularly useful in &#39;&#39;&#39;&#39;White light&#39;&#39;&#39;&#39; color printing methods.

United States Patent 1 Bey et al.

[ DIGITAL OPTICAL DENSITY FOR MEASURING THE DIFFERENCE IN OPTICAL DENSITY BETWEEN AN ABSORBER OF KNOWN DENSITY AND A SAMPLE ABSORBER [76] Inventors: Paul P. Bey; Michael P. Bey, both of 4909 Abbott Dr., Temple Hills, Md. 20031 [22] Filed: Aug. 25, 1971 [21] Appl. N0.: 174,827

Primary ExaminerRonald L. Wibert Assistant ExaminerV. P. McGraw Attorney-R. S. Sciascia et al.

[57] ABSTRACT An apparatus is provided for determining a difference [4 Oct. 16, 1973 in optical density between an absorber of known density and a sample absorber. An exponential decaying voltage is connected in series but of opposite polarity to a voltage source which is proportional to photodetector current flowing in a memory load. The current is generated by light of a specified wavelength passing through a standard optical absorber. The instant when the magnitude of the exponential voltage is equal to that generated by the photodetector, i.e., when the voltage across the combination is zero, the memory load is adjusted so that a specified number of clock pulses, N (fixed by the clock frequency and the time constant of the exponential voltage) are counted by a ripple counter. A sample of unknown optical density at a specified wavelength is then measured, providing a voltage proportional to a selected calibrated memory standard. At the instant of time when this voltage, in combination with the exponential voltage is equal to zero, the difference in the optical density between the unknown and standard absorbers is equal to the absolute value of the difference between the reference count N and the count in the ripple counter. The density information is gated into a decade counter and read out digitally.

7 Claims, 7 Drawing Figures RESET PULSE GENERATOR CORRECT INTENSITY COUNTER AND GATE LOGIC SAM PLE GATE G ENERATOR DIGITAL READ-OUT CIRCUIT INDICATOR I20 AUTOMATIC COLOR CO MPENSATOR I9) STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon of therefor.

BACKGROUND OF THE INVENTION The present invention relates to an apparatus and method for determining a relative difference in optical density between a known calibrated optical standard and a sample optical absorber, such as photographic film, and is specifically directed to a digital optical densitometer. I

The art of photography has long felt a need for an apparatus capable of quickly and accurately determining both positive and negative changes in the optical density of a sample absorber relative to that of a calibrated standard absorber. A major drawback in the prior art has been the fact that such devices lack accuracy and primarily, as typified by Bowker U. S. Pat. No. 3,528,749, most require a calibrated optical absorber simultaneously compared with the absorber under test throughout the operation'of the equipment. As can easily be seen, this arrangement is mechanically cumbersome and awkward sincethe accuracy of such a device substantially depends on the skill of the operator and his ability to attain proper placement of the absorber under test as well as perform an alignment of the standard absorber.

The existing problem becomes more acute when trying to apply the prior art equipment to the white light color printing method. In fact, until now, no equipment has provided means for efficiently applying this color printing method. In this method the white light is reduced to the primary colors of red, blue, and green, each color being individually and sequentially manipulated. The quality of the resultant photographic print depends to a large extent on the accuracy of matching the ratio of primary color intensities for a photographic film to be printed to that of a reference with the objective of making these ratios equal. If the ratio of the primary color intensities on the film do not differ from that of the reference, the resultantprint will accurately display the multitude of color combinations in the picture. Appropriate compensating filters must be used in order that the primary colors,'through a neutral part of thenegative, present a definite transmission. If the difference in density-for the primary colors relative to that of the reference 'areequal, the ratio of the primary color intensities are. also equaLTherefore, an apparatus capable of quickly determining the values of the compensating filters, which make the density .differences of the primary colors equal, is desirable.

Considering the drawbacks of the prior art, we have developed anapparatus capable of accurately measuring relative differences in optical density compensation by utilizing, among other things, a decaying exponential voltage source coupled with a memory circuit; all of which obviate the difficult calibration tasks of the prior art and the need for identical intensities for accurate density measurements.

SUMMARY OF THE INVENTION veloped by photocurrent through a memory reference load is made equal to the exponential voltage at a particular time t, which corresponds to a reference count N This photocurrent is generated by light of a specified wavelength passing through a reference absorber. A memory reference load bank is provided to relate the reference count to a multitude of different calibrated color standards. When the relative density of a sample absorber is to be measured, its corresponding color memory load is selected. A voltage proportional to the intensity of light at a specified wavelength is developed across the selected memory load and is compared to the decaying exponential voltage. At some point in time these two voltages will add to zero. When this occurs it is sensed and the time, as a function of counts in a ripple counter, is compared to the reference count N Thus a positive or negative difference in count appears between the standard and the sample. This count is gated into a decade counter. The difference in count is indicative of the density difference between the standard and the sample. The reset pulse is again triggered at the end of the density measurement to recycle the logic to provide substantially continuous readings. Also, intensity monitors are provided to assure that the intensity of the light required by the sample is within the operational tolerance of the system. If the intensity for a particular sample does not fall within the operating range an intensity control potential is varied to change the amplitude of the decaying exponential voltage without affecting the calibrated timing characteristics of the system.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to detect and indicate both positive and negative differences in density of an optical absorber relative to a selected standard absorber.

A further object of the present invention is to determine the relative density difference between two absorbers without regard to the specific value of their individual densities.

Another object of the present invention is to store many different density standards such that the density of each color to be measured can be compared to its individualprecise standard.

Another object of the invention is to provide density difference information in the form of a ratio so that parameters such as absolute intensity of the light source and intensity potentials do notaffect the measurement.

A further object of the present invention is to provide a densitometer capable of rapid updating so that accua following description taken in conjunction with the accompanying drawings in which the like reference numerals designate like parts and wherein:

DRAWINGS FIG. 1 is a functional block diagram depicting an embodiment of the invention;

FIG. 2 is a schematic diagram of an apparatus embodying the invention;

FIG. 3A is a graphical representation of an exponential decaying voltage due to a calibrated optical absorber of a specified color;

FIG. 3B is an exponential function for the measurement of a sample optical absorber;

FIG. 3C is an exponential voltagefor the measurement of a second sample optical absorber;

FIG. 4 is a graphical showing of the relationship be tween the decaying exponential voltage and the circuit logic for a positive density difference wherein the density must be subtracted to conform to the calibrated absorbet; and

FIG 5 is a graphical description of the relationship between the decaying exponential voltage and the circuit logic for a negative density difference wherein the density must be added to conform to the calibrated absorber.

DETAILED DESCRIPTION Referring to the block diagram in FIG. 1, optical density information is presented to sensor 103 through absorber n and color filter 117. Thesensor, a photomultiplier, provides a current directly proportional to the intensity of light impinging upon its surface from light source 101 after the light through color filter 117 of a specified wavelength )t, has passed through absorber n. Thus the current generated by the sensor, expressed as a function of density is:

where k is the sensitivity of the sensor, 1,, is the optical intensity of the color selectivity filter 117 for wavelength A I is the source intensity, and D, is the density of the absorber 20. The current 1,, generated by the sensor is applied to memory standard 104 which converts the current into a proportional voltage by the equation:

where R, is the resistance of the memory standard selected. Clearly the value of R, determines the value of current needed to produce a particular voltage.

The memory standards R, are variable resistors, such as R,, R,, R, and R, in FIG. 2. As an example assume R, corresponds to the memory reference load for a specified wavelength ).,.From equation (1) the reference current produced by the sensor .is:

re! A k(1)l0(D re!) where D,"" is the optical density of the reference optical absorber. From equation (2) the voltage impressed upon memory standard R, for this example becomes:

ral: rd RI Memory standard R, can now be adjusted to produce a precise voltage V," equal to V The memory reference load is designed such that a particular reference load R, can be selected and adjusted to provide a constant V developed by current If.

Considering any wavelength A, and the corresponding R, the same V can be obtained by the adjustment of the colors respective memory reference load thereby providing a density reference for as many colors as one wishes to calibrate. When an unknown color density for a particular color is to be measured, the re sultant photocurrent will generate a voltage equal to, less than, or greater than V depending on whether the density being measured is respectively equal to, greater than, or less than the particular calibrated density reference.

Applied in series with the voltage V,,, across the memory reference load 104, is'a decaying exponential voltage of opposite polarity generated by the sweep signal 106 and triggered by the reset pulse 110. The decaying exponential voltage V316 is developed to off-set the voltage V such that the resultant voltage V V q V,,. The amplitude of the exponential voltage, V,, is controlled by intensity control 108 which provides an adjustable d.c. level to the sweep circuit. The general equation for the decaying exponential is:

For a selected value of V,, the memory reference load R, is adjusted so that the voltage V,,""= V and V, 0 at a specified time Then equation (6) becomes:

as shown in FIG. 3A. At time t zero pulses are counted by the decade counter 112.

As shown in FIG. 38, a voltage V, which is smaller in magnitude than V, is generated by photocurrent produced by light of wavelength kn passing through an unknown absorber of density greater than that of the reference. Then V, 0 at a later time t,, as compared to such that Eq. (6) becomes subtracting equation Eq. (7) from Eq. (8),

-.=T1niv./v.1

Since [V,/V, l0[D D,,] (from Eq. (l-4)),t, t, is proportional to the difference in density between the reference and the unknown absorbers. The count in the decade counters is N-N, where N is the number of counts in the ripple counter at time t, and N, is the number of counts in the ripple counter at time t,. The difference in count is proportional to the increase in the unknown absorber density relative to that of the reference absorber.

If the density of the unknown absorber is less than that of the reference, V, is greater in magnitude than the exponential voltage V and the magnitude of the exponential voltage VRICl is equal to V,, at an earlier time compared to time as shown inFIG. 3C. The sensing of zero indicates the time t and t,-t is obtained. This difference in time is measured as N,,N. The time difference is proportional to the decrease in density of the unknown absorber relative to that of the reference absorber.

The time difference is then converted into decimal counts by the decade counter and gate logic 112 with time t corresponding to the particular reference count of zero. At the end of a measurement cycle, sample gate 114 triggers the digital read-out circuitry 118 providing useful density difference information to an indicator 120 and/or filter compensation system 122. Intensity indicators 116, inform the operator whether or not he has correctly adjusted V, intensity control 108.

Turning now to FIG. 2, an understanding of the circuit can best be made by setting forth the operation of the circuit under calibration procedures, and then considering the operation of the device when two different unknown color densities are to be measured.

CALIBRATION Current generated by photomultiplier sensor 103, produced by light of wavelength A, passing through an optical absorber, enters the circuit at point L. The voltage at point L can be written as follows:

where V is the voltage across the selected memory reference load R and:

R cf V0 p l l] Therefore from Eq.

L 0 I 1 1 a V is forced to zero by the decaying exponential, generated by sweep generator 106, at some time t= t so that V, V, exp [t'/R C 1' 2.31 R,c log [V,,/V

The time in units of the clock period can be expressed as where N is the number of pulses counted in time 1'.

To assure that a particular count N appears in the ripple counter at t t the time constant a-(R C and the clock frequency are chosen as follows:

From Eq. (15), t t can be expressed as:

t =N T The selection of N, depends upon the precision and range required for the device. For any range of density difference measurement AD, N would be chosen to equal (AD)(l0') andj selected equal to l, 2, 3... depending on the precision required (the precision increases with j). For a density difference range AD i 1.0, N, is chosen to equal 10, 100, or 10, (j l, 2, 3,...).

We have found that for a density range AD i L0 and for purposes herein N clock pulses (corresponding toj= 2), measured at time t,, provides sufficient precision for density measurements even though greater precision is possible.

Therefore from Eq. (16);

and from Eq. (7 and 14) we write:

100 T= t, 2.3] R C Log [V IV The clock frequency and the time constant R,C are selected such that:

10 100 2.31 R C /r,

for j 2. Substituting Eq. (19) into Eq. (18) 100 100 Log [V IV The variable resistor R,,, as well as each of the other memory reference standards R,,, can be adjusted such that V,," across these loads, for light of wavelength R, passing through a reference absorber, is equal to V and conforms with the proportion established by'Eq. (21

EXAMPLE I Assume that the density of the sample absorber is greater than the absorber of known density. The photocurrent then generates a voltage V, across the selected memory load which is less than V As can best be seen in FIG. 4, with reference made to FIG. 2, reset generator resets the count in the ripple counter to zero and sets X l in flip-flop 30. This starts the exponential sweep from generator 106 and simultaneously pulses are gated into the ripple counter 107. The sweep tinuousclockapulses from clock:109andthesignal Y from'flip-flop 40. As canbeseen fromFlG'. 2; .Y- will I remain -until:ripple counter l07counts100' clock pulses (6,Q,6,Q;Q C,6,Q,). Atsthe count of 100.,-

which corresponds to the "calibrated :reference time t NORgate 1,3'sets flip-flop 40 andas aresult Y:= lyY- will remain high until the next reset pulse appears- At the countiof100; with Y= l, ANDsgate -38ais no OR gate From equations (9) and (19),- theperiod of the clocktpulse-was'specificallyichosen to indicate a particular incremental-density difference;- Therefore each clock pulse which originates from AND gate 38, and gated'through' OR gate 15==-is proportional to the;

' density difference; Since -38' is1uninhibited at the count f of 100, or N,,, the'number of pulsescounted .by the de- 5 cade counters 42: and' 44 -will be counted with respect to the densityvreference; The clock pulses-arecounted. until timejt,,. when the magnitude of decayingexponentialyoltage equals V that :is'V 0 which-is sensed by zero sensing amplifier .The-. sen'sing;of :a -3 zero produces a pulse to'0Rgate 34, which in turnresets flipflop 30. The I resettingi ofiflip flopfio switches-X 0 which inhibits AhlD gate 38;- Thus, as'xcan readily be seen; clockspulses to thedecadecounters 42;.and Mare. stopped; and-thetrue: ;density difference. information,

correspondin'gi 'to a coiintof appears therein:

i I fromEqr-(9 and. i9);Attheiendzofthecount sample gate 114-1 pr' ovides ;a;:1pulse1 to quad-latch 46- and 48-,

' longer inhibited. Therefore clock pulses'are gated into erator 106, and simultaneously. pulses are gated into the-ripple. counter 2 10.7. I

Theilogici state; of X= 0 is appliedato .AND gate 54; Also. applied to. theinputs of AND gate 54are the. continuous clockpulses from clock109, land 1; When V 0 apulse .is generated byzero sensing amplifier 24; The pulse-from amplifier 24sets i=1; With 7 set higl'rfrom the reset pulse; AND? gate 54.1is no longer. inhibited, and as a result;clock; pulses are counted .bydecade. counters 42Jand 44; .The countk-in the decade counters continues until AND gate 54 :b'e- 1 comes inhibited by 1 At-the countof *100 (N5) :in the ripple. counter 107,. which-corresponds tov the calibrated-reference time t,, NOR gate 13 sets flip-flop 40 and as a result Tm-With 1 O AND'gate54ib ecomes inhibited and the. clock; pulses to the decade. counters 4 2i and 44 are, stopped-2 The number. of clo'ek pulses (which are incrementally proportional to the density) thatap'pearin the decadevcounters are-100 (N,) minus I the count in the rippletcounter at t as long as t, occursbefore. reference time .t Thus, as'canreadily be: seen,

the truedensity difference information; correspondingto thjecountof- N, appears in decadeacounters 42 and yielding japarallel shiftofinformation to decoder driver t 50: and-52; The-information becomes readable. as a re- Ysult of digital ji-ndicator l 1851f the: count :in .-the:ripple counter 102 reaches ZM b'efOre-a pulse. is: gerieratedby zero sensing'I-a'niplifier 24", Z,- is-s'et to; equal 1"which-pro.-

of the device.[

At'the end o' f'the count','*sample. gate l l' -t provides' pulseto q'uadlatch 46' and 48-, yieldinga parallelshift of information to decoder driver- 501and-i52'; (lf V- is greater-than V W is set'to ;l; which turns .on' lamp,v 47

indicatingthatthe measurement range of thedevice);

is. outside the preset In this 'example,,to ascertain that the count-is less thanthe. reference (indicating that thedensity ,isless); onemerelyhas .toexaminerthe state of flip-flop 26th this case when X goes from 1 l to"0,-.Y-: wasxequal to1-0.

' This puttA, 'and Af, high z.in latch"- 27'. correspondingly the combination of: lamps 2 9 and: 31; which are int the 1 t form of a. turn on.- .The' positive signindicate's-that I densityf' must be -added;totheabsorber ofunknown:

- duces'aj'pulse:tolORjateiM}which?resetsflip-flopylitl equaltothereferencefdem 'and; switches-X=0..Thisturns on-rangeylamp.53indi- W catirigthat the-measurementisoutsid e-thepreset range v In this example to 'ascertain that thefc'ount 'is-greater than the reference ceunr (indicatinglitha'tthedensity is greater) o neimerelyhas ,r exarni eihe state 3 of flip- I flop; 26.; in ,tliis'g case; .at theendkof the count; when: X i

went-from"lgtoxfiand: Y equals. 1. This combination'rsets A; zoflatch 27;; high; which turns. on' lampq29t' Lamp 29 is in the" ram of-sminussignto indicate,that density I v mustbeisubtr'actedfrorn-thefabsorber.of'unknowndensitytomake/fits density'equal: "to that .of' the: referenceabsorber;

voltageV Ascan bestibe 86en-iflFIGI '5 With reference made to FIG. 2, reset generatorl lo resets the count in the ripple counter 107'to zero andfX 01inflip-flop'30.

This starts 1theexponential sweep from generator 106;

andsiniulta n'eouslypulsesare gatedinto the ripple gendensity to make its density sity.

either subtracting. the; corresponding; density} difference asin-EX AMPLE I or by adding the correspond- Y ing-rdensity differenceasin" EXAMPLE I111, I It should. be'noted; that densitykdifference:measuremerits do notldependzon source intensity li orpotentisl V; as longas these .quantitiesremain; constant 'duringu f measurement.

We; will now demonstrate how adjustments of'Jyolt-I age, V and. the; source; intensities are utilized ;to*provideta minirrium number of compensatingfilterstomb tain a... correct :color balance for ai. negative to be printed; V Tov determinethe': densities of the a compensating; fil'.

ters for themeasuredrnegatiyetobe printed-.rtwotprecetions (standard-filter pack), which?" provides in. good; I colorbalancetor iii-reference negativewheniplaced 1a,;

jacent-to thewhitelight source is either l place.d;a d-

ja'cent to thewhitezlight sourceduring-d'e'nsit'y measurement of the measuredv'negative. or1(2) not adjscentsto the white-light source duringvdenlity measurementsz in thejfirstlcase, the filteripack for the negative to be printed is obtained by adding or subtracting the density measurements from the densities of the compensating filters in the standardfilter pack. In the second case, the color pack for the negative to be printed is determined directly by the density measurements.

In both cases calibration of the memory is made by measurements of the reference negative with the standard filter pack adjacent to the white light source. With V V the memory voltages VF are adjusted so that V (i a,b,c) so that a count of zero is obtained.

In general, where DJ (1' a,b,c) are the densities of the reference negative in combination with their corresponding compensating filters for wavelengths A compensating Filters of densities Df must be added or subtracted in order that,

I /I ref I /I re! I /I relf for a correct color balance of a measured negative. In Eq. (24), I, are the intensities of light transmitted through the measured negative or measured negative and standard color pack if the standard color pack is placed adjacent to the white light source and I! are the intensities of light transmitted through the standard color pack and reference negative.

Where K is any constant, Eq. (24) can be satisfied if: (1 For D K D,, a compensating filter of density DJ is added so that (2) For D1 K D a compensating filter of density D,-F is subtracted so that Measurement at wavelength X for the measured negative gives the result,

"2 "1 100 gic a a) if V,, V

We will consider the case where I'll and V IVJ' and then where T,, and T ref are transmissions through the reference and measured negatives, respectively, and K Log (C /C Similar expressions are obtained for wavelengths A, and h The time measured in units of clock counts in Eq. (29) is equal to 100X the density of the compensating filter (Eq. 25) to be added for a correct color balance of the measured negative.

The constants C, and C can be varied by adjustments of I and voltage V so that,

100K 100 (D DJ),

corresponding to zero correction for wavelength A The The compensating filters to add or subract for a color balance of the measured negative are those determined from the clock count for measurements at wavelengths and A providing a minimum number of compensating filters for a color balance of the negative to be printed.

It is believed that the inventive method and apparatus has been described with sufficient detail to enable one skilled in the art to practice the teachings contained herein. It is anticipated that many structural variations as well as electronic circuit equivalents may occur to those skilled in the art without there arising a departure from the spirit and scope of the invention.

What is claimed is: 1 A device for measuring a density difference between a known calibrated reference and a measured density comprising:

detecting means for providing a current proportional to an exponential of the measured density;

memory standard means having a plurality of individual memory loads connected to said detecting means for establishing a particular voltage corresponding to said current;

exponential sweep means connected to said memory standard means for generating an exponential sweep voltage wherein said exponential sweep means and said memory standard means together provide a voltage representing the sum of said particular voltage and the exponential voltage, the sum voltage being driven to a specific d.c. level;

d.c.-level-sensing means connected to said memory standard means for providing a pulse when said sum voltage is driven by said exponential sweeping means to said specific d.c. level;

counting means for determining a count indicative of a time difference between a calibrated point corresponding to the density of said known calibrated reference and said pulse;

digital read-out means connected'to said counting means for converting said count into usable digital data.

2. A device as claimed in claim 1 further comprising:

gating means incorporated in said counting means for providing information as to whether said count corresponds to a density less or greater than said calibrated reference;

indicator means connected to said gating means for displaying said information.

3. A device as claimed in claim 1 wherein said time difference is expressed as:

wherein r is a selected time constant of said exponential sweep means, and V,, is said particular voltage, and V is a voltage representing said known calibrated reference.

4. A device as claimed in claim 1 wherein said memory standard means are a bank of resistors, each corresponding to a different known calibrated reference wherein any one of said resistors can be connected to said detecting means at will. v

5. A device as claimed in claim 1 further comprising:

intensity control means connected to said exponening means is a photomultiplier.

237 33 I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION I Patent No. I 3, 765 778 Dated I October 16, 197-3 Inventor(s) Paul P. Bey and Michael P. Bey

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Title, after "Density" insert Device Signed and sealed this 23rd day of April 197A.

(SEAL) Abte st:

EDWARD I-.I.FLETCHI'IR,JR. C MARSHALL DANN Attesting; Officer Commissioner oi Patents MELVIN L, CRANE Acting Patent Counsel zg g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 765778 Dated October 16, 197-3 Inventor(s) Paul P. Bey and Michael P Bey It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Title, after "Density" insert Device Signed and sealed this 23rd day of April l97L (SEAL) Attest:

EDWARD I-I.FLETGl-IEE-,JR. G MARSHALL DANN Attesting Officer Commissioner of Patents MELVIN L. CRANE Acting Pat-ent- Counsel 

1 . A device for measuring a density difference between a known calibrated reference and a measured density comprising: detecting means for providing a current proportional to an exponential of the measured density; memory standard means having a plurality of individual memory loads connected to said detecting means for establishing a particular voltage corresponding to said current; exponential sweep means connected to said memory standard means for generating an exponential sweep voltage wherein said exponential sweep means and said memory standard means together provide a voltage representing the sum of said particular voltage and the exponential voltage, the sum voltage being driven to a specific d.c. level; d.c.-level-sensing means connected to said memory standard means for providing a pulse when said sum voltage is driven by said exponential sweeping means to said specific d.c. level; counting means for determining a count indicative of a time difference between a calibrated point corresponding to the density of said known calibrated reference and said pulse; digital read-out means connected to said counting means for converting said count into usable digital data.
 2. A device as claimed in claim 1 further comprising: gating means incorporated in said counting means for providing information as to whether said count corresponds to a density less or greater than said calibrated reference; indicator means connected to said gating means for displaying said information.
 3. A device as claimed in claim 1 wherein said time difference is expressed as: Tau 1n (VR/Vn) wherein Tau is a selected time constant of said exponential sweep means, and Vn is said particular voltage, and VR is a voltage representing said known calibrated reference.
 4. A device as claimed in claim 1 wherein said memory standard means are a bank of resistors, each corresponding to a different known calibrated reference wherein any one of said resistors can be connected to said detecting means at will.
 5. A device as claimed in claim 1 further comprising: intensity control means connected to said exponential sweep means for varying the magnitude of said exponential sweep.
 6. A device as claimed in claim 1 further comprising: latch means connected to said counting means for holding said count; sampling means connected to said latch means and said digital read-out means for a variable rate of sampling said count wherein said latch means holds said sample means intermittently to sample said count and present said count to said digital read-out means.
 7. A device as claimed in claim 1 wherein said sensing means is a photomultiplier. 